1. Field of the Invention
The present disclosure relates to surge protection elements and semiconductor devices.
2. Description of the Related Art
Gallium nitride (GaN)-based wide bandgap semiconductor has a dielectric breakdown field strength higher than that of semiconductor such as silicon (Si), and a saturated drift velocity of electrons higher than that of compound semiconductor such as gallium arsenide (GaAs) or Si semiconductor. Accordingly, the GaN-based wide bandgap semiconductor is expected to serve as materials for a power semiconductor device which conducts large current with high breakdown voltage. In particular, in a hetero structure of AlGaN/GaN, electric charges are generated on a (0001) plane of the GaN layer at the interface between the AlGaN layer and the GaN layer by spontaneous polarization and piezoelectric polarization. This provides a sheet carrier concentration of 1×1013 cm−2 or higher and a high mobility of 1000 cm2 V/sec or higher even if the AlGaN layer and the GaN layer are undoped. Accordingly, a hetero-junction field effect transistor which uses two dimensional electron gas at the heterointerface provides a low on-resistance power transistor.
For the purpose of use as a power control device, there is a need for a normally-off GaN-based hetero-junction field effect transistor which interrupts current between source and drain at the gate voltage of 0 V similarly to a conventional Si-based power MOS transistor. Such a normally-off GaN-based hetero-junction field effect transistor can be provided by including a structure in which a p-type semiconductor layer is disposed between a gate electrode and an AlGaN layer, as disclosed in Patent Literature (PTL) 1.
Although the GaN-based hetero-junction field effect transistor has excellent device characteristics as described above, the structure disclosed in PTL 1 has a problem in that the avalanche resistance which is an index indicating the robustness of a power device is extremely low. A transistor having a low avalanche resistance easily breaks down upon application of a surge voltage exceeding the rated breakdown voltage of the transistor.
In order to improve the avalanche resistance of a GaN-based transistor, PTL 2 discloses a technique in which a diode including Si having a high avalanche resistance is integrated on a substrate of the GaN-based transistor. Specifically, PTL 2 discloses a structure in which current is actively conducted through the diode at the time of avalanche current conduction so as to improve avalanche resistance substantially.